It is known to transmit digital data by providing a differential phase shift to a continuous wave (CW) signal, wherein the phase shift is keyed to the data value. Such data transmission and decoding systems are known as DPSK (differential phase-shift keyed) systems, and include DPSK encoders, transmitters, receivers and decoders for encoding, transmitting, receiving and decoding the data. The present invention is directed to the DPSK decoder, only, independently from the other components of such a system.
In typical prior art systems, such decoders operate using analog techniques. More particularly, the prior art uses analog phase locked loops, multipliers, dividers, phase detectors, filters and comparators to provide a demodulated signal. However, such systems suffer from various deficiencies known in the art, including a requirement for a large number of parts, susceptibility to temperature changes and resultant variation of system characteristics, and general variation of characteristics with aging of the components. Thus, the decoding efficiency and hardware reliability of such prior art systems is adversely affected by use of analog technology therein.
Decoding efficiency and reliability are extremely important in various applications of DPSK systems, however. For example, where DPSK data is used in a microwave landing system to guide an airborne vehicle, the consequences of a system error may be catastrophic. There is accordingly a need in the prior art for a DPSK system having improved decoding efficiency and improved reliability.
Yet another problem encountered with the prior art systems is the difficulty of selecting components to compensate for variations in signal frequency, as well as the above noted effects of temperature and component aging. Accordingly, there is a further need for a DPSK decoder having easily replaceable and controllable components, wherein changes of various parameters may be simply and easily compensated for, whether by replacement of a component or by automatic changes in system operation to provide such compensation.
In typical prior art systems a CW signal is derived from the modulated input signal for comparison with the received signal. In these prior art systems, a squaring loop is used wherein the received, modulated, signal is multiplied by itself, thus providing a double frequency, unmodulated, signal. To use this signal as a reference for comparison with the modulated signal, it is necessary to divide the frequency by two in order to provide a reference having the same frequency as the modulated signal. Comparison of the reference signal and the received (modulated) signal provides an indication of the phase shift therebetween, and thus of the data value used to provide the indicated phase shift.
As above noted, the prior art systems utilize analog components for performing the above functions, and are thus subject at each step of the decoding process to the various difficulties characteristic of analog circuits.
There is accordingly a need for DPSK decoders having an improved feedback control circuit to provide compensation for various errors and parameter changes which occur during operation. There is also a need for providing increased reliability for known DPSK decoders, of the type available in other areas by using digital techniques and digital circuitry.